266 NEMBRTEA. 



body at the anterior opening, and becomes external when the proboscis 

 is projected by eversion (Fig. 219 Us, B). The eversion is no doubt 

 caused by the contraction of the muscular wall of the sheath bringing 

 pressure to bear upon the contained fluid, and, when completed, the 

 middle portion of the retracted proboscis is at the front end of the 

 protruded organ. That is to say, the hinder part of the proboscis is 

 not eversible : it lies within the protruded part of the proboscis in 

 the projected state, and in the hinder part of the sheath in the 

 retracted state. This non-eversible portion has glandular walls and 

 contains a fluid. In the armed forms (Hoplonemertini) it is separated 

 from the eversible part by a contraction of its cavity, which almost 

 closes up the eversible part, and a stylet, to which may be added two 

 groups of small accessory reserve stylets, is placed at this point. 

 The posterior part of the proboscis opens by a narrow aperture at 

 the base of this stylet, and allows the contents of the glandular 

 non-eversible part, which is very possibly poisonous, to exude. 

 When the proboscis is completely everted the stylet projects freely 

 at its apex. 



In the unarmed forms (Anopla) the stylet is absent, but the 

 surface of the proboscis which becomes external on eversion possesses 

 numerous nematocysts. The proboscis is therefore clearly offensive, 

 but very possibly it also possesses a tactile function. 



The body-wall varies in structure in the different orders. It 

 always possesses an external layer of ciliated ectoderm cells con- 

 taining mucus -secreting gland-cells. Within this there is in the 

 Heteronemertini (1) a thin basement membrane, (2) a cutis con- 

 taining connective tissue and some longitudinal muscular fibres, (3) 

 a layer of longitudinal muscular fibres, (4) a thin layer of a plexiform 

 nervous matter containing at two points the lateral nerve-cords, (5) 

 a layer of circular muscular fibres, and (6) of longitudinal muscles; 

 this is followed by a compact mass of reticular connective tissue 

 which passes into the muscular coats of the intestine and proboscis 

 sheath. In other Nemertini there are only an external circular and 

 internal longitudinal muscular layer. All the layers above mentioned 

 are embedded in a gelatinous albuminoid material, which is especially 

 developed in the transparent pelagic form, Pelagonemertes, and is of 

 the same nature as the jelly of Medusae. There is therefore no 

 body cavity in ISTemertines, though, as in Rhabdocoeles, spaces in 

 the connective tissue may in some cases be so large as to simulate 

 one. 



The nervous system consists of two cerebral ganglia connected 



